1. Field of the Invention
This invention relates to methods and apparatus for detecting signals below a noise floor or weak signals around a detection threshold, particularly signals from quiet radars and direct sequence radar modulations, otherwise known as low-probability-of-intercept (LPI) radars. Many of these radars employ pulse compression and power management to avoid ESM (Electronic Support Measures) detection.
More particularly, the present invention relates to circuitry which can detect, demodulate and characterize bipolar phase shift-keyed (BPSK) signals with much less complicated circuitry than that previously used.
2. Background Art
Low probability of intercept radars try to do what their name implies, namely reduce the available peak power of the radar signal without corrupting the target detection process. As a target approaches closer to the LPI radar, the radar processor dynamically reduces the power on the target to keep the signal-to noise ratio (S/N) of the detection power constant. A classic technique is to use pulse compression, where the energy is spread out but recovered by the time bandwidth of the receiver. Sometimes binary phase shift keying is used to randomly change the phase within a long pulse for added protection from electronic countermeasures (ECM).
Present LPI demodulation techniques employ complicated synchronizing techniques and carrier regeneration circuits to demodulate the baseband. Also phase-locked-loop circuits are needed to recover the carrier frequency.
FIG. 1 is a simplified block diagram showing the basic elements of a BPSK transmitter. An RF carrier is input to a modulator M. A pseudonoise signal is generated by a PN generator G, which may be a conventional SAW-type PN generator. The PN signal is a string such as 01101110... and is supplied to an IF input of the modulator M. An example of an output signal is shown in FIG. 2. As seen therein, a long radar pulse P is composed of a bipolar RF signal S having several phase reversals R which are responsive to the modulating PN signal.
U.S. Pat. No. 4,647,863 dated Mar. 3, 1987 to Skudera and Konig (incorporated by reference) illustrates means for detection of PSK signals. The present disclosure will apply a technique disclosed in the '863 patent for detection and demodulation of complex LPI radar modulations, such as BPSK signals.
According to a method of detection disclosed in the '863 patent, to detect BPSK, a synchronous detector is used with a delay corresponding to the highest chip rate, i.e., the frequency of the BPSK signal. For a pseudo-random or pseudo-noise (PN) sequence, one half of the highest chip rate is used. Usually this technique is used with IFM (Instantaneous Frequency Measurement) receivers, also known as DFD (Digital Frequency Discriminator) receivers, and requires very high signal-to-noise ratios.
See also U.S. Pat. No. 4,443,801 to Klose and Skudera, and U.S. Pat. No. 4,965,581 to Skudera and Albert, both incorporated by reference, which are of background interest.